U.S. Pat. Nos. 4,138,879 and 4,235,095 to Liebermann disclose systems for detecting inhomogeneities in a fluid within a conduit. In these systems a pair of electromechanical transducers are disposed on a fluid-filled conduit in an acoustically coupled relationship. An adjustable-gain driving amplifier responsive to the output of one transducer for driving the other transducer has its gain automatically adjusted to maintain the system on the margin of oscillation. An indicating circuit detects modulation of the driving signal. Inhomogeneities such as bubbles passing through the conduit near the transducers cause variations in the gain required to maintain the system on the margin of oscillation and are detected as modulations of the driving signal.
For many applications of the device disclosed in the above-mentioned patents there are no stringent requirements for the manner in which the acoustic energy is coupled from the transducer to the conduit. This is generally true where the conduit has a very large diameter-to-wall thickness ratio or when only certain modes of oscillation of, for eample, a pipe wall will occur. In such applications the transducers may be coupled to the conduit or pipe by coupling members of simple construction. For example, the transducers may be mounted on thin metal plates which are secured in an apparatus for holding these plates in contact with the conduit wall. Alternatively, the transducers may be glued to a portion of a metal or plastic clamp fastened directly to the outside of the conduit by means such as a spring or thumb screw. Acoustic energy passes directly through the clamp to the conduit wall.
In many applications the detecting apparatus becomes insensitive to bubbles if the transducer mounting means described above is clamped in a slightly different configuration on the same conduit. Random displacement followed by reclamping sometimes restores sensitivity. These insensitive configurations are accompanied by modes of oscillation which are generally much higher, but sometimes lower in frequency than the more sensitive modes. These undesirable oscillations represent vibrational modes of the conduit and coupling member system in which acoustic energy travels primarily in the conduit, rather than in the fluid.
An electrical filter is not a desirable solution to this problem because the coupling member and pipe each contribute to the mode of oscillation. An electrical filter inserted into the system would have a fixed frequency which would not readily accommodate changes in conduit size, clamping configuration and other application parameters.